Airborne contamination in the form of large particles (e.g. greater than 10 .mu.m) is difficult to measure due to known limitations associated with sampling. As particle size increases, the distribution of particles entrained in a given air flow changes due to the momentum of the larger particles' mass overcoming the entraining forces of the air flow (i.e., the number of particles of a given size may decrease, thereby changing the distribution). Since the entraining forces are not large enough to change the particle direction, impaction of particles on the sampling device's inlet nozzle tends to occur. The impaction phenomenon versus particle size skews the results and, thereby, the counting efficiency.
Particle impaction phenomenon is distinct, however, and can therefore be used as a sampling technique in and of itself. At least one device utilizes large particle impaction for measuring aerosols. The device is manufactured by Berkeley Controls Inc., of Laguna Beach, California, having a model designation C-1000A QCM. The device may be considered a cascade impactor which creates an air flow through a system of impactor plates. Each impactor has a certain aerodynamic design which allows particles of less than a certain aerodynamic diameter to "follow", or be entrained by, the air flow through the device and not be impacted. Particles above the certain aerodynamic diameter do not follow the air flow path and are impacted on an impactor plate. Therefore, each of the plates are designed to capture particles of a certain size and above. In the case of the above identified model, there are 11 cascaded impacters, each with its own size limit for impaction.
The concentration of particles in the cascade impactor is determined by measuring the frequency response of crystals, located on the impactors. The mass of the impacted particles on the impactor plate causes the crystals' frequency to shift, from which certain information regarding the particles may be determined. Although this particular device has the advantage of being able to measure the concentration of particles while sampling (i.e., real time measurement), most impactor style devices are designed such that the impactor plate must be weighed after a sample is taken in order to calculate a concentration.
In general, impactor style devices have several disadvantages. First, size is a disadvantage. If real time measurements are needed, the device is typically quite large. Second, the impactor surfaces become contaminated with impacted particles and need to be cleaned and serviced regularly. Third, when an impactor is used, there arises the need to pull a particle laden sample airflow through the device. In many cases pulling an artificial sample airflow disturbs particle distribution and skews the results. Fourth, the device has an inherent inability to measure particle concentrations accurately due to the finite resolution of the particle impaction system.
U.S. Pat. No. 4,739,177 describes another type of large particle detection sensor, offering many advantages over the above described impactor design. The disclosed sensor bounces a collimated laser beam back and forth a selected number of times between two mirrors such that the beam creates a light net. The beam is terminated at the end of the system. The sensor has an open cavity defining a flow volume through which the test sample passes, with the mirrors being located on two opposing edges of the flow volume. The area between the mirrors defines a sampling volume, with the area covered by the laser beam defining a viewing volume. Particles which pass through the viewing volume intersect the beam, thereby scattering light. The scattered light is collected and converted to a corresponding particle size based on the collected amount of scattered light.
This type of sensor offers several advantages over the above described impactor designs. First, it has an open cavity (i.e., flow volume) for particle detection and, therefore, needs no pump for pulling a sample through the sensor. Second, it does not use an impactor design and, therefore, does not need frequent cleaning and servicing.
An example of the type of sensor disclosed in U.S. Pat. No. 4,739,177 is manufactured by High Yield Technology, of Mountainview, California, designated model PM-100. The sensor exhibits a disadvantage in that the beam of light does not cover one-hundred percent of either the flow or sampling volumes. Instead, the sampling volume only covers 50% of the flow volume. Further, the viewing volume is only 36% of the flow volume. Therefore, only 18% of the sample aerosol moving through the sensor's flow volume is actually sampled.
Another disadvantage of this device is the limited counting efficiency of particles actually in the viewing volume. Only 13% of the 6 .mu.m particles in the viewing volume are counted due to the fundamental operating limits of the device (i.e., scattered light collection limitations, among others). Therefore, the device's theoretical efficiency is only 4.7% for 6 .mu.m particles passing through the sample volume (See Rob Caldow, Performance of The High Yield Technology PM-100 Particle Flux Monitor, M.S.M.E. Thesis, (1987)).
A third disadvantage of this latter style sensor is that it collects the scattered light from the particles. In optical particle counters, scattered light is considered to be the light which is directed out of the path of the beam. As disclosed in U.S. Pat. No. 4,739,177, scattered light has many angular dependencies and thus causes a signal output which is not a linear function of particle size.
Light extinction, however, is considered to be the light which is removed from the beam path. The extinction function for large particles is quadratic with respect to the particle's diameter, or is a linear function for the projected area of large particles and thus allows for a more accurate device in large particle size ranges. Further, extinction techniques are less sensitive to stray light from other sources than are scattered light devices. Still further, use of extinction eliminates the need for collector optics.
Therefore, as can be appreciated, there arises a need for a method and device for producing real time, continuous measurements of particles. This device should allow for maximizing actual sampling of the flow and sample volumes and for maximizing counting efficiency of the particles moving through the sample volume. The device should also detect single particles, include means for solving inherent sampling problems, and allow for a large fluctuation in sample flow rates without adversely affecting the device's particle detection characteristics.